1. Field of the Invention
The present invention relates in general to antenna systems mounted on motor vehicles, and more particularly, to electronically controlled antenna systems of a type which can change the antenna operation mode in accordance with the surrounding condition.
2. Description of the Prior Art
One of conventional electronically controlled antenna systems of the above-mentioned type is disclosed in "IEICE (INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS) TECHNICAL REPORT" Vol. 89, No. 250 RCS89-31 issued in 1989 from THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS. The system shown in the publication is of a so-called "multiplex wave suppressing adaptive array type". In the antenna system of this type, a plurality of antenna elements are arrayed, and operation weights of the elements are electronically controlled by a signal processor to direct the directivity of the antenna toward the transmitting source (or station) of the desired radio wave. That is, in this system, delayed radio waves which lower the quality of radio wave communication is suppressed. The signal processor comprises a frequency A/D (analog/digital)-converting section which is associated with each antenna element, a frame synchronizing section which selects, among signals received by the antenna elements, the most powerful signal and carries out a frame synchronization on the received signals, and a digital signal processing section which synthesizes the branched signals.
The antenna system of this type can exhibit a satisfied performance in an area, such as the suburbs, wherein the direction in and from which the desired radio wave comes to the antenna is generally constant. However, when the motor vehicle having such antenna system mounted thereon comes to an urban area where a plurality of large buildings stand close together, the system fails to exhibit the satisfied performance because the direction of the desired radio wave is caused to change at frequent intervals.